1. Field of the Invention
The present invention relates in general to weld process monitoring techniques, and more particularly to improved methods for real-time monitoring of thermal radiation of a weld pool to monitor a size variation and a focus shift of the weld pool for weld process control, utilizing the chromatic aberration of focusing lens or lenses.
2. Description of the Prior Art
The application of high power Nd:YAG lasers for precision welding in industry has been growing quite fast these days in diverse areas such as the automobile, the electronics and the aerospace industries. Nowadays, a Nd:YAG laser with as much as 10 kW of average power is available in the market and the fiber delivery of it makes it useful for many remote applications. On the other hand, these diverse applications also require the new developments for the precise control and the reliable process monitoring. Due to the hostile environment in laser welding, a remote monitoring is required and man acoustic and optical remote monitoring techniques have been developed. However, acoustic monitoring is not suitable for the application in a factory due to the acoustic interference from environmental noise. Therefore, optical monitoring is preferred in industrial applications.
In a laser welding, a laser beam is focused on a workpiece by a focusing lens or lenses. The focusing lens or lenses image an aperture liming the size of the laser beam on the workpiece and the size of focused laser beam is the image size of the aperture on the workpiece at the wavelength of the laser. A weld pool is generated by the interaction of the focused laser beam and the workpiece. Due to the thermal conduction of the workpiece, the size of the weld pool is generally not the same as the size of the focused laser beam and varies with the power of the laser or with the focus shift of the focusing lens or lenses. The weld pool radiates a thermal radiation. Many optical monitoring methods have been developed measuring the thermal radiation from a weld pool.
As to the optical monitoring, two approaches have been followed: one monitors the image of a weld pool with a CCD or an infrared camera and the other monitors the thermal radiation from a weld pool with one or more single-element detectors. The monitoring of image requires the fast data processing and is quite complicated and expensive to be implemented. Furthermore, the monitoring of image is not applicable to a laser welding with a laser delivery fiber because the image of a weld pool can not be transmitted through a single-core laser delivery fiber. On the other hand, the monitoring of thermal radiation is simple and cheap to be implemented, and fast and robust for industrial applications. However, the information on a weld pool status in the thermal radiation monitoring is limited compared to the image monitoring. Therefore, several spectral bands of thermal radiation from ultraviolet to infrared have been monitored with a plurality of detectors to widen the information on a weld pool status. Examples of such method for weld monitoring can be found in U.S. Pat. Nos. 4,446,354, 5,155,329, 5,272,312, 5,360,960, 5,506,386, 5,651,903, 5,674,415, 5,681,490 and 5,728,992.
However, these methods could provide some information on the change in the status of a weld pool, but could not provide the information on the focus shift of a focusing lens or lenses to maintain a uniform laser welding. On the other hand, chromatic aberration of a lens or lenses has been used in the focus control of a lens or lenses as shown in U.S. Pat. No. 4,992,859 and has also been used in the distance measurement or in the distance control as shown in U.S. Pat. No. 5,785,651 or in U.S. Pat. No. 5,218,193. In the monitoring of a laser welding, U.S. Pat. No. 5,850,068 could provide the information on the focus shift of a focusing lens or lenses using the chromatic aberration of the focusing lens or lenses by subtracting one spectral band signal from the other spectral band signal. However, U.S. Pat. No. 5,850,068 could not provide the information on the size variation of a weld pool. Furthermore, the information on the focus shift of a focusing lens or lenses provided by U.S. Pat. No. 5,850,068 would be incorrect if the power of a laser is varied because the subtraction of one spectral band signal from the other spectral band signal is doubled if the thermal radiation intensity is doubled by the increase of laser power even if the focus position of a focusing lens or lenses is not shifted. The information on the focus shift of a focusing lens or lenses provided by U.S. Pat. No. 5,850,068 would also be incorrect if the size of a weld pool is varied during a laser welding because the focus position of the highest intensity of each spectral band signal shifts as the size of a weld pool varies and the error signal changes even if the focus position of a focusing lens or lenses are not shifted.
The variation of a laser power can be easily monitored at the laser unit itself, but it is not easy to monitor the variation of a laser power at a workpiece due to the absorption by mirrors or lenses in the path of laser delivery. Therefore, a focus shift monitoring independent of the size variation of a weld pool is required for industrial laser welding applications.
As to the image monitoring of a weld pool using a CCD or an infrared camera, the size of a weld pool provides important information on the status of a weld pool. It can provide the information on the change of a laser power at a workpiece and also the information on the weld depth. If a variation of weld pool size is measured, the variation of weld pool size can be compensated by adjusting the laser power. However, the size of a weld pool measured with a CCD or an infrared camera depends on the focus shift of a workpiece. If the focus position of a workpiece is shifted during a laser welding, the size of a weld pool measured with a CCD or an infrared camera varies and the information on the status of a weld pool becomes incorrect. Therefore, a size monitoring of a weld pool independent of the focus shift of a workpiece is required for industrial laser welding applications.
U.S. Pat. No. 5,875,026, Korean Pat. No. 0193,276 and Japanese Pat. 2,895,021 disclosed a method measuring the size variation and the focus shift of an extended radiation source using the chromatic filtering wherein the chromatic aberration of imaging optics is used to provide the information on the size variation and the focus shift of an extended radiation source by measuring spectral band signals of thermal radiation through the imaging optics and through an aperture. The variations of the transmittances of the spectral band signals through the imaging optics and through the aperture are used in providing the information on the size variation and the focus shift of an extended radiation source.
U.S. Pat. No. 6,188,041 used the chromatic filtering disclosed in U.S. Pat. No. 5,875,026, Korean Pat. No. 0193276 and Japanese Pat. 2895021 to provide the size variation and the focus shift of a weld pool in a pulsed laser welding wherein the size of a weld pool could be reduced between the laser pulses due to the thermal conduction cooling through a workpiece. In U.S. Pat. No. 6,188,041, an algorithm was disclosed wherein a function obtained from the transmittances of the spectral bands for the size measurement of a weld pool provided a reference size which is usually the same as the size of a focused laser beam so that the size of a weld pool could be measured from the reference size by comparing a value obtained from the processed spectral band signals with another value of the function obtained from the transmittances of the spectral bands for the size measurement and another algorithm was disclosed wherein another function obtained from the transmittances of the spectral bands for the focus shift measurement of a weld pool provided the information on the focus shift of a weld pool by comparing a value obtained from the processed spectral band signals with another value obtained from another function obtained from the transmittances of the spectral bands for the focus shift measurement. In U.S. Pat. No. 6,188,041, the wavelengths of spectral bands and the timings to measure the spectral band signals were optimized to measure the size variation of a weld pool independently from the focus shift of a weld pool and to measure the focus shift of a weld pool independently from the size variation of a weld pool. These features were possible because in a pulsed laser welding the size of a weld pool is reduced between the laser pulses due to the thermal conduction cooling through the workpiece. However, in a continuous laser welding, the size of a weld pool is generally larger than the size of a focused laser beam and does not vary much except at the beginning of a laser welding. Therefore, the algorithms and the optimization in the selection of the wavelengths of spectral bands in U.S. Pat. No. 6,188,041 could not be applied in a continuous laser welding. In U.S. patent application Ser. No. 09/589,134 and Korean Pat. Appl. No. 10-2000-0029230, the algorithms and the optimization in the selection of the wavelengths of spectral bands were generalized to be applicable to a continuous laser welding if the curvatures and materials of focusing lenses were known. However, the curvatures and materials of focusing lenses are usually not known in industrial laser welding applications and the development of a method to overcome this problem is required in industry.
Hence, it is the fundamental object of the present invention to provide a method whereby an independent monitoring of the size variation and/or the focus shift of a weld pool in a laser welding, wherein the curvatures and materials of focusing lenses are unknown and the chromatic aberration can not be calculated, can be obtained in a manner which is simple and suitable for industrial applications. It is another object of the present invention to provide a method wherein a uniform laser welding is obtained by compensating the size variation with a laser power control and by controlling the position of a weld pool in focus position.
These objects are satisfied by utilizing the chromatic filtering of the thermal radiation of a weld pool. A method for monitoring the size variation and the focus shift of a weld pool is provided for a laser welding wherein the size variation of a weld pool is monitored independently from the focus shift of a weld pool and the focus shift of a weld pool is monitored independently from the size variation of a weld pool. In a laser welding, a weld pool is generated on a workpiece by transmitting a laser beam through an aperture which limits a size of a laser beam and focusing transmitted laser beam with at least one lens with some chromatic aberration but minimum spherical aberration on a workpiece. The thermal radiation from a weld pool is measured at three spectral bands through at least one focusing lens and through the aperture which limits a size of a laser beam or through any other aperture which limits a size of a weld pool wherein the thermal radiation is measured with single-element detectors after the thermal radiation is separated from the reflected laser beam with a dichromatic mirror, after splitting the spectral bands of the thermal radiation with dichromatic mirrors and beam splitters and after filtering each spectral band with a narrow band-pass optical filter. A distal end of an optical fiber can be used as an aperture for a laser deliverable through an optical fiber.
In a laser welding, a laser beam is focused on a workpiece by a focusing lens or lenses. The focusing lens or lenses image an aperture limiting the size of the laser beam on the workpiece and the size of focused laser beam is the image size of the aperture on the workpiece at the wavelength of the laser. A weld pool is generated by the interaction of the focused laser beam and the workpiece. Due to the thermal conduction of the workpiece, the size of the weld pool is generally not the same as the size of the focused laser beam and varies with the power of the laser or with the focus shift of the focusing lens or lenses. The weld pool radiates a thermal radiation according to the blackbody radiation law and the spectral dependence of the thermal radiation can be estimated. Due to the chromatic aberration of the focusing lens or lenses, the transmittance of each spectral band of the thermal radiation varies with the size variation and with the focus position of a weld pool and the spectral band signals measured with single-element detectors vary if the size and/or the focus position of a weld pool varies. The transmittance of a thermal radiation on a weld pool through focusing lens or lenses and through an aperture can be calculated as a function of position on the weld pool at each spectral band if the curvatures and materials of the focusing lens or lenses are known for a plurality of focus shifted positions of the weld pool. Furthermore, the spectral band signals follow the blackbody radiation law so that the dependence of the spectral band signals on the size variation and the focus shift of a weld pool can be estimated using the transmittance functions of the spectral bands.
For a focus shift monitoring, the ratio of two spectral band signals, one in the shorter wavelength than the laser wavelength and the other in the longer wavelength, is used wherein the transmittance functions of the two spectral bands are as same as possible so that the focus shift monitoring becomes independent from the size variation of a weld pool. However, the transmittance functions can not be calculated if the curvatures and materials of focusing lenses are not known. This invention provides a method to select the wavelengths of two spectral bands for a focus shift monitoring experimentally.
On the other hand, for a size variation monitoring, the wavelength weighted three spectral band signals, one in the shorter wavelength than the laser wavelength, another in the longer wavelength and the other near the laser wavelength, are used wherein the focus shift dependence of the size variation monitoring obtained from the spectral band signals near the laser wavelength and in the shorter wavelength is compensated by the focus shift dependence of the size variation monitoring obtained from the spectral band signals near the laser wavelength and in the longer wavelength, but the sensitivity of the size variation monitoring is enhanced by optimizing the wavelengths of the spectral bands so that the size variation monitoring becomes independent from the focus shift of a weld pool. The compensation parameter can be calculated if the curvatures and materials of focusing lenses are not known. This invention provides a method to determine the compensation parameters experimentally.
In conclusion, the monitoring of the size variation and the focus shift of a weld pool is achieved with a plurality of single-element detectors by utilizing the chromatic filtering of the thermal radiation from a weld pool. The use of a plurality of single-element detectors rather than a CCD or infrared camera makes it very fast to process the data and cheap to be implemented for industrial applications. The monitoring of weld pool size variation can also be used to monitor the weld depth in a laser welding. Furthermore, the monitoring of the size variation of a weld pool is independent from the focus shift of a weld pool and the monitoring of the focus shift of a weld pool is independent from the size variation of a weld pool. In other words, the simultaneous monitoring of the size variation and the focus shift is achieved. The methods developed in the present invention for monitoring the size variation and the focus shift of a weld pool can be applied to both a continuous laser welding and a pulsed laser welding.
These and other features, aspects and advantages of the present invention will become better understood with preference to the following description and appended claims.